Levine, J.M. 2011. Science.

Species diversity and biological invasions: Relating local process to community pattern.

Reviewed: 09/23/11

The question of species diversity and its effect on the invasibility of communities is confounding and not yet fully determined. This paper attempts to show some of the finer points of this complexity using a natural-system based approach to looking at diversity and invasion. The set-up is a riparian system in California where tussocks form from the common sedge Carex nudata. These tussocks form miniature islands of varying size that provide structure and habitat for native and invasive species. The author saw that tussocks with higher diversity had an increasing likelihood of the presence of three invasive test species. This observational test was performed on tussocks of similar size. The study does not include counts of invaders present, simply presence.
The next experiment was designed to determine if the observed pattern was simply a function of invaders desiring similar environmental conditions as the natives, or if diversity itself was the cause. All species were removed from test tussocks and replanted with a range of 1 to 9 native species out of a pool of 9 species. After a primary growth season, seed of all the invaders was added to each tussock. The results showed that species richness had an effect on invader success. The r-squared values were low however and all invaders were seeded together on each island, ignoring possible confounding effects of invader-invader interactions.
The author provided one further test to account for the covariance of several variables, such as percent shading, disturbance, and number of propagules via downstream delivery of the river. Richness was only able to account for 25% of the explained variation in invader success. The final experiment showed that invasion success was not altered by richness if counts of initial invader propagules was high.
Taken together these results illustrate a key point: that species diversity has an important alleviating effect on invader success at the local/neighborhood scale, which was easily defined by the boundaries of the tussocks. However, at the community scale, the same processes that drive high diversity, namely downstream delivery of seeds, or covary with high diversity, may be responsible for driving the establishment and survival success of invaders upwards. Leading to the main conclusions that more diverse communities may be more invasible, but a decrease in diversity, particularly at the local scale, may also allow invader success.